Ecotones, or broad-scale boundaries between biological communities, generally include structural and taxonomic elements from both adjacent communities. Research has explored factors that can create or maintain ecotones, as well as ecotonal influences on habitat quality and fluxes of matter, energy and organisms. However, it remains uncertain how disparate, spatially juxtaposed elements function and interact within the ecotone itself. Do ecotones represent a functional melding of the adjacent communities, or do different components merely overlap in space while continuing to function as they would in their ‘home’ communities? We investigated long-term plant community responses to fire and grazing in a 7000 km2ecotone between mixed-grass prairie and sagebrush steppe, two of North America’s most dominant ecosystems. These vegetation types have dramatically different historical fire regimes, resistance to grazing, and invasibility by annual grasses. We asked whether plant community responses to disturbance within the ecotone were more similar to mixed-grass prairie, sagebrush steppe, or a unique melding of these two ecosystems. We sampled plant community composition at 70 pairs of transects located inside and outside of wildfires that burned from 1937-2012, as well as 11 pairs of transects located inside and outside of grazing exclosures maintained for >50 years.
Results/Conclusions
Plant community responses to wildfire and grazing exclosures did not vary substantially across the broad study region, despite continuous variation in shrub cover. Woody plant responses were characteristic of sagebrush steppe. Burned transects had <10% as much shrub cover as unburned transects. Shrub regeneration rates were slow, and burned sites were often colonized by grasses. Grazing exclosures had larger, but not more numerous shrubs. In contrast, the understory plant community behaved very similarly to a northern mixed-grass prairie. Recently burned sites had high forb cover and were not invaded by annual grasses. Over time, burned sites were colonized by perennial grasses and continued to resist invasion. Long-term grazing exclosures had high cover of C3 perennial grasses and weedy annuals, including the invasive grass Bromus tectorum. A similar response to the long-term absence of grazing has been observed in multiple other long-term grazing intensity studies from mixed-grass and shortgrass prairie ecosystems. Our results suggest that within an ecotone, different components of the plant community can maintain functional fidelity to their ‘home’ ecosystems, despite being spatially juxtaposed. The idea of overlapping, rather than intergrading, plant communities may provide new opportunities for targeted management within ecotones.